Particle Control: PlasmaSurface Interactions

1
Particle Control Plasma-Surface Interactions

• Effects of impurities in Tokamak
• Plasma sheath theory
• Plasma-Surface interacting processes
• Atomic and molecular processes
• Desorption Wall conditioning Techniques
• Sputtering
• Arcing
• Evaporation
• Particle control in tokamak
• Limiters and Divertors
• Scrape-off layer
• Recycling

2
Effects of impurities in Tokamak

• Radiative power loss line radiation
• Fuel dilution
• Radiation barrier difficult to heat plasmas
  initially
• Disruptions via edge cooling

3
Basic Concepts of Plasma Sheaths sheath
formation

• Plasma sheath the non-neutral potential region
  between the plasma and the wall caused by the
  balanced flow of particles with different
  mobility such as electrons and ions.

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High electron mobility --gt negative potential
buildup

• High energy ion bombardment
• Electrons are retarded
• Ambipolar diffusion established

4
Basic Concepts of Plasma Sheaths presheath
formation

• Presheath a transition layer between the
  neutral plasma and the non-neutral sheath in
  order to maintain the continuity of ion flux,
  giving rise to an ion velocity at the
  plasma-sheath edge known as the Bohm velocity uB.

nenino
neni
ns
ni
x
ne
presheath
sheath
plasma
plasma
presheath
sheath
Sheath edge
0
s
x
s
0
5
Bohm Sheath Criterion
Electron density in Boltzmann distribution
Ions entering into the sheath with velocity vo
Ion density in the sheath from constant ion flux

Electric potential at sheath by Poissons
equation
Bohm sheath criterion
for small potential near sheath edge
Bohm velocity--gtsound speed
6
Presheath and Sheath Potentials

• Potential drop across the presheath accelerating
  the ions to the Bohm velocity
• where ?p is the plasma potential
• with respect to the sheath-presheath potential.

• Substituting for the Bohm velocity

plasma potential

• Density at the sheath edge to that in the plasma
  from Boltzmann relation

Sheath potential at a floating wall from the
ambipolar diffusion condition where the mean
electron velocity,
wall potential
Solving for the wall potential ?w ,
including secondary electron emission effects
total secondary emission coefficient, ?
7
Plasma Ion Energy at the Surface
thermal energy sheath potential
Acceleration by sheath
ion flux density
sheath power transmission factor
8
Plasma-Surface Interacting Processes

• Atomic and molecular processes
• Desorption Wall conditioning techniques
• Sputtering
• Arcing
• Evaporation

9
Atomic and Molecular Processes

• Atomic reactions
• excitation H e --gt H e
• ionization H e --gt H 2e
• charge exchange H H --gt H H
• Molecular reactions
• dissociation H2 e --gt H H e
• dissociative ionization H2 e --gt H H 2e
• H2 e --gt H H 2e
• molecular ionization H2 e --gt H2 2e
• dissociative recombination H2 e --gt H H

10
Atomic and Molecular Processes

• Relative reaction rates depend on plasma
  temperature and density
• Rate coefficients for hydrogen atoms and
  molecules

11
Atomic and Molecular Processes

• Ionization and charge exchange influence the
  transport of recycling species and impurity
  species
• Charge exchange dominant hydrogen processes
  random walk diffusion
• Ionization dominant impurity ions are multiply
  ionized
• Dominant charge states of the impurity
  determined by electron temperature, electron
  density, and residual time
• Photon efficiency
• ion influx with absolute radiation
• average energy loss per ionization
• Inverse photon efficiency

12
Impurity Ion Temperature
Calculated temperature of some typical impurity
ion species as a function of background plasma
temperature

• Thermalization time

• Ionization time

• low temperature the impurities are quickly
  thermalized with low ionization rates
• high temperature ionization occurs before
  thermalization

13
Charge State Distribution of Impurity Ion Species
Local electron temperature determines the charge
state
Oxygen ionization state distribution in coronal
equilibrium
14
Adsorption and Desorption

• Adsorbed atoms hydrogen, carbon monoxide,
  water, etc
• weakly bound physical adsorption 0.3eV
• strongly bound chemical adsorption 3eV
• Desorbed by incident ions, neutrals, electrons
  and photons
• electron and photon processes electronic, weak
• ions and neutrals by momentum transfer, strong
• cross section ? up to 10-18m2, yield

surface concentration

• Desorption can lead to
• impurity accumulation in the plasma
• lack of density control when plasma species
  desorbed

Incident ion flux density
need wall conditioning
15
Energy Dependency of Desorption Cross Section
4He incident, CO on nickel
3He incident, H on tungsten
4He incident, H on molybdenum
H incident, D on nickel
16
Wall Conditioning

• Baking the vacuum vessel, typically to 200-350C
• Discharge cleaning
• surface cleaned by particle bombardment in
  discharges
• glow discharges effective and simple, combined
  with RF operating at lower pressure of 0.1Pa
• pulsed discharges tokamak ohmic discharge w/o
  TF
• ECR discharges resonance location can be varied
• enhanced cleaning with hot vessel with less
  readsorption
• light ions such as hydrogen(with chemical
  action) and helium(remove oxygen and hydrogen
  with carbon walls) are used to avoid sputtering
• Gettering wall covered with a metal film by
  evaporation
• Carbonization and boronization covering wall
  with low Z

• Wider operating range up to higher densities w/o
  excessive radiation
• High density and low temperature decrease
  sputtering yields

not applicable for reactor
17
Gettering with Thin Metallic Film
Wall covered with a clean metal film by
evaporation

• remove unwanted impurity species fresh layers
  of chemically active metals react with active
  gases such as O2, CO, H2, and CO2 binding them
  tightly to the surface
• reduce outgassing sequential deposition bury
  the adsorbed gases

• Materials for gettering
• high chemical reactivity and high vapor
  pressures at modest temperatures, typically
  1500-2000ºK titanium, chromium
• beryllium good getter, low atomic number, but
  high toxicity

• Disadvantages
• should cover at least 30 of the vacuum vessel
  surface
• quick saturation and need getter between shots
• film flakes with the size of 10-100?m random
  impurity injection

18
Carbonization and Boronization
Cover the tokamak wall with low Z non-metallic
films(C B) to minimize the release of high Z
impurities

• Carbonization
• gaseous carbon compound(CH4) --gt glow discharges
  --gt deposit a thin layer of amorphous carbon on
  the wall (optimum temp. 300ºC)
• initially increasing the hydrogen --gt make
  density control difficult --gt recycling control
  with helium glow discharge after carbonization
• optimum thickness for good adhesion 1?m --gt
  short lifetime

• Boronization
• similar to carbonization with boranes(B2H4 ,
  B2H6) at 400ºC --gt boron acts as getter and thin
  boron films pump oxygen and hydrogen
• Trimethyl borone, B(CH3)3, forms mixed films of
  carbon and boron
• low affinity of boronized surface for water
  vapor(good for opening)
• silane(SiH4) deposit Si film good getter, but
  higher atomic number
• disadvantages toxicity of both borane and
  silane

19
Sputtering
Removal of atoms from the solid surface by the
impact of ions or atoms, resulting in impurity
radiation and surface erosion

• Sputtering yields
• decreases with increasing sublimation energy
• increase with increasing energy transfer

reflection
Threshold energy
m1,2 masses of incident and target atoms

• Sputtering yields simulated by Monte Carlo code
• linearly increases after threshold until
  saturated
• decreases at higher energy since collision
  cascade occurs away from the solid surface in
  deeper location
• maximum yield move to higher energy as target
  mass increases
• magnitude of sputtering yield depends on surface
  binding energy
• surface structure and impurity level can change
  the binding energy

20
Energy Dependence of Sputtering Yield
General semi-empirical curve for sputter yield
yield factor depends on incident and target atom
combination
nuclear stopping cross section
Thomas-Fermi energy
threshold function

• Sputter yield influenced
• by incident angles affected by the ion Larmor
  radius, sheath acceleration, and
• the surface roughness

21
Energy Distribution of Sputtered Atom

• most probable energy 0.5Es (2-3eV)
• energy distribution varying as E-2 at high
  energies
• higher mean energy when sputtered by heavier ions

22
Sputtering Models

• flow balance in steady-state

• energy balance

radiated power
input power
energy transported to the surface per e-i pair
23
Choice of Materials

• impurity production rates
• structual strength
• neutron activation
• thermal shock resistance

minimize Z and sputter yield

• figure of merit

maximum allowed impurity concentration
plasma sputtering coefficient
plasma edge temperature
24
Arcing
Sustained with low voltage, high current
Joule heating, evaporation erosion

• Power arc by external potential
• Unipolar arc by plasma sheath

Ion currents 7-10, 50-100eV, in charge states
up to 4-5
25
Heat Flux, Evaporation, and Heat Transfer

• evaporation
• --gt erosion, contamination
• --gt low vapor pressures
• low sputtering yield
• thermal shock
• --gt loss of structual strength
• --gt high thermal conductivity

Upper limit of tolerable heat flux 10-20MW/m2
Heat flux for high reliability 2-5MW/m2
26
Particle Control in Tokamak

• Last Closed Flux Surface(LCFS) determined by
• Limiters
• Divertors
• Scrape-Off Layer
• Recycling
• Tritium Behavior

27
Limiters define plasma boundary

• Roles of the limiter
• protect the wall from the plasma disruptions,
  runaway electrons, other instabilities --gthigh
  heat loads --gt refractory material
• localize the plasma-surface interaction
• localize the particle recycling high neutral
  density and radiation

• Material selection criteria for the limiter
• withstand thermal shock
• produce as low an impurity flux as possible
• maintain low atomic number with impurity
• have good thermal conductivity for heat transfer

• Materials for the limiter
• low Z materials carbon and beryllium, high
  heat loads
• high Z materials tungsten and molybdenum, good
  thermal properties, low sputtering yields
  however, very low concentrations allowed because
  of their high Z

28
Limiters

• Different types of limiters have different
• connection lengths
• scrape-off layer decay lengths

For long pulse/steady state operation, thermal
capacity become important
toroidal limiter(spread the heat load) or
divertor(impurity shielding)
29
Divertors define the LCFS solely by the magnetic
field and isolate plasma surface interactions
from the confined plasma

• Possible ways of reducing power density at the
  target
• placing the target tiles at an oblique angle to
  the field lines
• flux expansion of the field lines as they
  approach the target
• magnetically sweeping the strike point over a
  width gt ?p
• radiating power before reaching to the target by
  conduction
• transferring the energy to neutral particles in
  the divertor

Avoiding target surface erosion as well as
impurity flow into plasmas

• Objectives of divertor design in the fusion
  reactor
• minimizing the impurity content of the plasma by
  having the plasma surface interactions remote
  from the confined plasma and designing the
  divertor particle flow
• removing the alpha particle power by heat
  transfer through a solid surface to a fluid
  transfer medium
• removing the helium ash resulting from the
  fusion reactions

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Scrape-Off Layer radial distribution
In steady-state, particle balance gives
with scrape-off thickness, or e-folding length,
for density
Similarly, electron heat balance gives where
Cross field diffusion coefficient
Cross field thermal diffusivity
31
Scrape-Off Layer global balance
Global particle and energy balance total
particle out flux total flux to limiter
simple edge transport model for ?p
ionization rate coefficient
initial neutral velocity
flux e-folding length
32
Parallel Transport outside the LCFS
Isothermal fluid model
For steady-state, inviscid, isothermal, 1-D flow,
particle and momentum conservation gives
Mach number
so that
density at stagnation point
Plasma potential by considering Boltzmann
distribution of electron density
Flow velocity is difficult to calculate and there
is little experimental information
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One-dimensional Fluid Model of Divertor SOL
Assume

• no energy or momentum sources or sinks
  (radiation) in the scrape-off layer
• Simplified geometry between X point and the
  target
• Energy flow from the confined plasma

Momentum conservation
Heat transport along the SOL electron heat
conduction
For constant q//,
Power density transmitted across the plasma
sheath at the target
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Solutions
Target temperature
When sufficiently large temperature drop, i.e.
35
Radial Power Distribution in the SOL
Steady state power flow in the scrape-off layer
using
setting
when
for
Poloidal heat flux
36
Volume Losses of Power in the Divertor
To minimize power deposition on the target
plates, radiate power so that it can be
distributed over a large surface area

• Introduce impurity to enhance the radiation,

maximum radiation parameter, R(Te) 10-31Wm3,
for 1GW radiated power, nm neVlt 1040m-3
nm /ne10 with ne 1020m-3 and V 10m3
Lead to impurities flowing into the confined
plasma Cause unacceptable increase in the target
sputtering

• Volume loss mechanisms with charge exchange
  neutral loss (low plasma temp.) and ion-neutral
  collisions (high neutral density)

• Detached divertor plasma momentum and energy
  must be transferred from the plasma to a neutral
  gas blanket near the target
• Detached plasma drops target density --gt
  difficult helium ash removal

37
Flow in the Divertor

• Ionization due to recycling is localized near
  the target --gt density peaks and temperature
  falls
• Helium ash removal requires very high pumping
  speed --gt transporting the plasma to the separate
  divertor chamber can ease the restrictions
  (central fueling with NBI and pellets)
• High ionization due to high local density --gt
  reverse flows back to LCFS

38
General Design Considerations for the Divertor

• Single and Double nulls
• double null doubles wall interaction area and
  halves connection length, more triangularity,
  decreases plasma volume
• Target geometries flat plates and enclosed
  chamber
• flat plates simple, easy diagnostic access,
  rigid structure
• enclosed chamber good isolation from the main
  confined plasma
• Target tiles
• reduce thermal stress due to non-uniform heat
  flux --gt make small
• increase the effective area with small angle,
  and displace targets
• Erosion of the surface and consequent
  redeposition of eroded material

39
Recycling
Recycling each plasma goes to the divertor
target plate or limiter and returns to the plasma
many times during the discharge
Recycling coefficient ratio of the returning
flux to the plasma from the solid, to the
incident flux
Efficient recycling coefficients with additional
influx from adsorbed particles ( gt1)

• Particle backscattering coefficients, Rp
• Energy reflection coefficients, RE

40
Recycling backscattered ion energy distribution

• Backscattered particles are predominantly
  neutral
• Average energy depends on RE/Rp

Hydrogen diffusion in solids - exothermic
trap - endothermic escape
Rate coeff. of thermal desorption
Rate coeff. of entering trap
b.c.
41
Tritium Behavior
Diffusion-dominated hydrogen distribution

• implanted tritium moves both by diffusion and
  surface recombination

• release rate for diffusion dominant case with
  uniform distribution

• non-metalic material porous, pearmeate and
  trapped at the lattice defects

--gt heating and hydrogen discharge can remove
tritium

• Wall materials(exothermically dissolving
  hydrogen, Ti, Zr, Nb) release little gas and
  build up tritium inventory --gt not tolerable